Magnetic particle imaging of macrophage phagocytosis evaluates plaque vulnerability and therapeutic efficacy in atherosclerosis

Background Developing new atherosclerosis therapeutics remains challenging, with few drugs reaching clinical use. A non-invasive imaging method to assess plaque vulnerability is needed to provide early efficacy readouts and help select promising candidates for clinical trials. Magnetic particle imaging (MPI) is a novel tomographic imaging method, which directly quantifies the superparamagnetic iron oxide nanoparticles via its magnetization response, offering a linear signal-concentration relationship, background-free ‘hot-spot’ imaging eliminating tissue interference, and ultra-high sensitivity. Our prior MPI work sensitively evaluated vulnerable plaque features, including myeloperoxidase and hemorrhage in animals. In this study, we aimed to identify key markers of macrophage phagocytosis linked to plaque vulnerability and evaluate whether MPI of phagocytic activity can detect therapeutic responses and changes in plaque instability. Methods Integrated transcriptomic and immunofluorescence staining analyses of human carotid plaques were conducted to identify key phagocytosis-related markers. RAW264.7 macrophages were incubated with a novel synthesized polyethylene glycol-coated superparamagnetic iron oxide nanoparticle (PESPION) or comparator iron oxide agents (Vivotrax, Ferumoxytol). Vulnerable plaques were induced in apolipoprotein E (ApoE)⁻/⁻ mice by cast placement over the carotid artery to evaluate in vivo phagocytic activity using MPI. After 10 weeks of treatment with a proprotein convertase subtilisin/kexin type 9 inhibitor (inclisiran) or a sodium-glucose cotransporter-2 inhibitor (empagliflozin), macrophage phagocytosis and several features of plaque vulnerability were assessed by PESPION-enhanced MPI and histopathology, respectively. Results Macrophage scavenger receptor 1 (MSR1) was identified as a key phagocytosis marker for plaque vulnerability. Compared with Vivotrax and Ferumoxytol, PESPION demonstrated superior, concentration-independent iron uptake in macrophages, which was blocked by an MSR1 antibody or inhibitor. In vivo, PESPION enabled sensitive MPI detection of vulnerable plaques, with signals substantially reduced upon MSR1 inhibition. Both inclisiran and empagliflozin considerably reduced MPI-quantified phagocytic activity compared to controls, correlating with improved plaque stability confirmed histologically. MPI signals correlated positively with plaque area and macrophage infiltration, negatively with smooth muscle cell content, and strongly predicted the vulnerability index. Conclusions MSR1 is a key mediator of phagocytic activity in plaque vulnerability. PESPION-enhanced MPI provides as a sensitive, non-invasive tool for quantifying MSR1-mediated macrophage phagocytosis, enabling the robust evaluation of plaque vulnerability and pharmacological response.